Zemanek, U.S. Pat. No. 3,369,626 discloses an ultrasonic tool for use in scanning the inner surface of an open well borehole or of casing in a borehole. The tool, which is commercially known as the "borehole televiewer", creates a high resolution picture of the inner surface under investigation. The borehole televiewer is used to "see" the inner surface under investigation through drilling mud or other borehole fluids. In an open borehole, the borehole televiewer provides a picture of the formations surrounding the borehole. In a cased borehole, the borehole televiewer provides a picture of the inner surface of the casing, which can be used to determine the condition of the inner surface.
The borehole televiewer uses a rotating ultrasonic transducer. The transducer serves as a transmitter, to generate acoustic waveforms, and a receiver, to receive the acoustic return. The acoustic return is caused by the reflection of the generated acoustic waveform from the inner surface under investigation. The acoustic return has two measured parameters, the time of travel of the acoustic return and the amplitude envelope, which give an indication of the condition of the investigated surface.
The transducer rotates about three revolutions per second, is pulsed about 500 times per revolution, and is pulled up the borehole at a speed of about 5 feet per second. The ultrasonic transducer spot size, the rotational speed, the pulse repetition rate, and the vertical speed combine to provide full coverage of the investigated inner surface, resulting in high areal resolution of the inner surface.
In the borehole televiewer, the acoustic return is converted into a voltage by the transducer and sent to the receiver. The receiver, which is located downhole inside of the borehole televiewer, has a resistive network for attenuating the signal. It is believed that one of the primary functions of the attenuation resistive network is the protection of the receiver electronics during the high voltage excitation waveform which is used to generate an acoustic waveform from the transducer during the transmit period. Because the same transducer is shared by the transmitter and the receiver, some type of input protection for the receiver is necessary. However, use of an attenuation resistive network reduces the overall signal-to-noise ratio and requires additional amplifier gain in order to compensate for the attenuation.
Another disadvantage in the prior art receiver lies in the use of a bulky electromechanical rotary switch to change the attenuation, and therefore the gain of the receiver. The switch setting is selectable by an operator on the surface. The operator issues commands to the electromechanical rotary switch. The commands are sent down to the switch over the logging cable. As the switch rotates through its various combinations, the operator determines the most suitable setting for the well being logged. A relatively large amount of time is required to change attenuation setting, with the result being that the same receiver gain must be utilized for several acoustic returns and gain switching between parts of an individual return sequence is impossible. Yet, the acoustic returns obtained during a single attenuation setting may differ considerably in amplitude. Acoustic returns from relatively close inner surfaces will have a larger amplitude than acoustic returns from more remote inner surfaces. Rapid changes in amplitude are encountered at or near the edges of pits, holes, or corroded areas. It is unlikely that the operator will be able to identify a single gain setting that would provide satisfactory resolution of signals from both close and remote inner surfaces. Thus, there frequently arises the situation where a suboptimal log of the investigated inner surface is obtained. The operator must either then make multiple logging passes using different receiver gain settings and thereby increase logging time, or be satisfied with the suboptimal log.